A capacitive passenger detector is described in JP-A-2008-203150 corresponding to U.S. patent application Publication No. 2008/0198024. The detector includes a capacitive sensor. The capacitive sensor includes a main electrode disposed on a seat of a vehicle, a sub electrode disposed on the same seat, and a guard electrode disposed just below the main electrode. The sub electrode is spaced apart from the main electrode, and the sub electrode and the main electrode are arranged along with a front-rear direction of the vehicle. The guard electrode is arranged between a body of the vehicle and the main electrode, and spaced apart from the main electrode. The body is electrically coupled with a ground of the vehicle.
When the capacitive sensor measures a capacitance of the passenger on the seat, the main electrode and the guard electrode have the same electric potential. Thus, an electric field is formed between the main electrode and the body so that the capacitance of the passenger is detected. Specifically, the capacitive sensor detects disorder of weak electric field generated between the main electrode and the body so that the capacitive sensor outputs a current or a voltage corresponding to the detected disorder, i.e., a change of the electric field. Based on the current or the voltage, a passenger detection ECU as an electric control unit determines whether the passenger sits down in the seat.
In the above detector, as shown in FIG. 1, a passenger detection ECU 1 is coupled with a capacitive sensor 2. When a capacitance of a passenger in a vehicle is detected, a main electrode and a guard electrode have the same electric potential. When a potential difference between the main electrode and the guard electrode is zero, an output current from the main electrode is the same as an output current from the guard electrode. Thus, the current from the main electrode and the current from the guard electrode do not interact with each other. Thus, a proper capacitance CH of the passenger is obtained. Here, a reference CMG in FIG. 1 represents a capacitance between the main electrode and the guard electrode. A reference CB represents an error capacitance between the guard electrode and a body of the vehicle. A reference CH represents a capacitance of the passenger. A reference Y1 represents an output current corresponding to the capacitance CH of the passenger. A reference Y2 represents an output current corresponding to the error capacitance CB.
However, in the capacitive passenger detector, the area of each electrode is enlarged so as to increase a detection sensitivity of the capacitive sensor 2. Accordingly, the capacitance CH between the main electrode and the guard electrode increases. Thus, it is difficult to control the potential difference between the main electrode and the guard electrode to be the same potential with using the passenger detection ECU 1. Accordingly, when the potential difference between the main electrode and the guard electrode is generated, the output current from the main electrode is different from the output current from the guard electrode. Thus, the output current from the main electrode and the output current from the guard electrode interact with each other so that they interfere with each other. Thus, as shown in FIG. 2, a current shown as an arrow Y3 in FIG. 2 flows from the main electrode to the guard electrode. Alternatively, a current shown as an arrow Y4 in FIG. 2 flows from the guard electrode to the main electrode. Thus, the current Y3, Y4 affects the detection current for detecting the capacitance of the passenger so that the detection current is changed. Thus, the ECU 1 cannot detect the proper capacitance of the passenger.